1. Field of the Invention
The present invention relates to a method and apparatus for improving transmission time interval bundling in a wireless communication system, and more particularly, to a method and apparatus for enhancing an efficiency of transmission time interval bundling and preventing from wasting radio resource.
2. Description of the Prior Art
Long Term Evolution wireless communications system (LTE system), an advanced high-speed wireless communications system established upon the 3G mobile telecommunications system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B alone rather than in Node B and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE, a technique of transmission time interval (TTI) bundling is introduced for improving uplink coverage according to the prior art. TTI bundling is performed by repeatedly transmitting a same packet with default times, and those repeatedly transmitted packets are named a bundle. UEs in cell boundary utilizing TTI bundling can reduce transmission delay and signaling of control channels for enhancing reliability and accuracy of transmission, to improve LTE uplink coverage.
TTI bundling is performed under the existing system, to repeatedly transmit each packet for a fixed times. Under such circumstances, since a length of TTI and an operation of Hybrid Automatic Repeat Request (HARQ) are unchanged, the prior art has provided different solutions targeting on TTI bundling.
The first solution is: a network end generates an acknowledgement signal according to a decoding result of the last packet of a bundle. Therefore, even if other packets of the bundle are received successfully, the UE has to retransmit the bundle once the last packet is not received successfully. This unnecessary retransmission causes waste of radio resource and affects transmission rate. In addition, a process of bundle retransmission lacks efficiency. For example, please refer to FIG. 1A, which is a schematic diagram of packet transmission and reception of a UE. Assuming that the length of TTI is T, and HARQ_RTT represents HARQ round trip time (RTT) with a fixed length 8T. Packets PI1, PI2, PI3 and PI4 are the same packets transmitted repeatedly for 4 consecutive TTIs from a UE, which means the packets PI1, PI2, PI3 and PI4 form a bundle BDL. When the UE finishes transmitting the bundle BDL at time point 4T, the network end generates an acknowledgement signal (ACK/NACK) according to a decoding result of the last packet P4 of the bundle BDL to indicate the reception status of the bundle BDL. In this case, the packet P4 is not received successfully by the network end, so the network end reports a non-acknowledgement signal NACK. When the UE receives NACK from the network end between time points 7T and 8T, the UE cannot retransmit the bundle BDL at a starting point of the next HARQ_RTT (at time point 8T) since the processing time is too short. As a result, the UE starts to retransmit the bundle BDL after waiting for a HARQ_RTT, namely at time point 16T. Simply speaking, the UE receives the acknowledgement signal from the network end at the fourth TTI after transmitting the bundle BDL, and determines whether the bundle BDL needs to be retransmitted. If the bundle BDL needs to be retransmitted, the UE has to wait for a HARQ_RTT to retransmit the bundle BDL. Under such circumstances, the unused HARQ_RTT, which is from time point 8T to 16T, is not utilized for packet transmission, becomes waste of network resource, and affects transmission efficiency.
The second solution is: a network end generates an acknowledgement signal according to a decoding result of the first packet of a bundle. This solution can improve the problem that the UE retransmits the bundle BDL after waiting for a HARQ_RTT as shown in FIG. 1A. However, this solution can still cause unnecessary retransmission. For example, in FIG. 1B, when the UE finishes transmitting the packet PI at time point IT, the network end generates an acknowledgement signal (ACK/NACK) merely according to a decoding result of the packet P1 to indicate a reception status of the bundle BDL. If the packet P1 is not received successfully by the network end, the network end will report NACK. When the UE receives NACK from the network end between time points 4T and 5T, the UE can retransmit the bundle BDL at a starting point of the next HARQ_RTT (at time point 8T) since the processing time is enough. Therefore, such solution improves the transmission efficiency of the bundle BDL, and allows the UE to retransmit the bundle BDL without waiting for a HARQ_RTT. However, because the network end determines whether to report NACK only according to the decoding result of the packet P1, unnecessary retransmission may occur if only the packet P1 is not received successfully while the others are received successfully, causing waste of network resource and affecting the transmission rate.
The third solution is similar to the first solution, namely, a network end generates an acknowledgement signal according to a decoding result of the last packet of a bundle, and the difference is that the length of HARQ_RTT is no longer fixed to 8T, but is prolonged to 12T according to a length of the bundle, which is shown in FIG. 1C. When the UE finishes transmitting the bundle BDL at time point 4T, the network end generates an acknowledgement signal (ACK/NACK) according to a decoding result of the last packet P4 of the bundle BDL to indicate the reception status of the bundle BDL. In this case, the packet P4 is not received successfully by the network end, the network end reports NACK. When the UE receives NACK from the network end between time points 7T and 8T, the UE can retransmit the bundle BDL at a starting point of the next HARQ_RTT (at time point 12T) since HARQ_RTT is prolonged to be 12T. In other words, the UE receives the acknowledgement signal from the network end at the fourth TTI after transmitting the bundle BDL, and determines whether the bundle BDL needs to be retransmitted or not. If the bundle BDL needs to be retransmitted, the UE can retransmit the bundle BDL at the next HARQ_RTT (from time point 12T) since HARQ_RTT is prolonged to 12T. However, although this solution improves the shortcomings in FIG. 1A and shortens packet latency, the length of HARQ_RTT is prolonged according to the length of the bundle, causing transmission time to increase. Moreover, different lengths of bundles makes different lengths of HARQ_RTT, which easily causes difficulties in implementation and does not conform to a universal specification.